1. Field of the Invention:
This invention relates to a multiple-beam laser and more particularly to gas lasers capable of producing multiple independent pulsed laser beams, with the same or different wavelengths, at relatively high pulse repetition rates.
2. Description of Related Art:
There are a number of applications requiring laser beams that can be changed quickly from one wavelength to another. For example, in the remote detection of gases, such as chemical agents in the atmosphere, a line-tunable pulsed CO.sub.2 laser can be arranged to probe the absorption spectrum of the trace gas to be detected.
In such a remote sensing system, a laser pulse at a first CO.sub.2 line is transmitted through the atmosphere in the direction of the target area to be searched. The diffused back-scattered laser energy from a remote topographical target, or from atmospheric aerosoles, is detected by a receiver at the laser transmitter location. In the well-known differential LIDAR or DIAL measurement, the transmitted pulse is first tuned to a CO.sub.2 laser line at a wavelength near coincidence with an absorption line of the trace gas to be searched for. The laser wavelength is subsequently switched to another CO.sub.2 laser line at a wavelength removed from the absorption lines of the trace gas. The intensities of the two back-scattered or reflected signals from the target area are compared as a measure of the degree of absorption at the absorption line of the trace gas and therefore as a measure of the particular trace gas present in the atmosphere. For more detailed diagnostic and identification purposes, and to distinguish from spurious gases that might interfere with the identification, it is necessary to probe several absorption lines of the trace gas it is desired to detect.
Lasers using one adjustable mirror grating in which the wavelength can be manually adjusted by changing the angle of the grating are well-known. However, to minimize variations caused by atmospheric turbulence between successive pulses it is desirable that the successive pulses at different wavelengths occur within some relatively short time interval. The maximum acceptable time interval is so short as to render manually tunable laser systems impractical.
One known method for rapid wavelength tuning utilizes a rotating grating in a wavelength tunable grating resonator. The grating is used in Littrow for operation at a given laser line. In Littrow, a collimated ray at one particular wavelength incident on the grating is reflected from the grating along a direction exactly opposite from that of the incident collimated ray. Wavelength selectivity comes about because in Littrow the angle of incidence is dependent upon wavelength. The position of the rotatable mirror is reused in the course of its rotation by firing the pulsed laser at a moment when the grating is position to cause laser action at a preselected CO.sub.2 laser line.
In practice because of the high inertia of the relatively massive grating required for use with a high-intensity laser, the grating is mounted in a fixed position, but its image is moved by changing the angle of a mirror positioned to intercept the laser beam. In such a system, the resonator consists of one fixed mirror, one rotating mirror and one fixed grating. As the mirror rotates, the angle of incidence on the grating changes causing the wavelength at which the Littrow reflection occurs to change correspondingly.
The position of the mirror rotation is reused by reflection of a collimated light, such as a He-Ne laser beam, from the rotating mirror, and the use of an array of linear detectors. In practice, this method can be used for wavelength switching at a relatively slow speed of about one-fiftieth of a second. This limitation is because of the practical difficulties in extracting a fast rise electrical signal to reuse the position of the rotating mirror. Although it is possible to fire the laser repetitively on the same pulse line, corresponding to the same mirror position, from pulse to pulse, there is difficulty in switching the operating laser to another preselected line. This is in part because the build-up of energy in different lines occurs with different time delays with respect to the trigger pulse which switches on the laser.
Instead of a continuously rotating mirror, a torque mirror can be used to turn the mirror, but bring it to rest under the control of an electrical signal. The laser is fired while the mirror is in a fixed position. This arrangement eliminates the problem caused by the time delays of the energy build-up. The process can then be repeated by choosing the next line and turning the mirror to the corresponding position. That solution, however, is limited by the practical capabilities of existing torque motors. The mirror must have a relatively large mass or it will suffer distortion during the deceleration that brings it to a halt. The resulting inertia of the mirror limits the switching speed, using commercially available torque motors, to 5 to 10 milliseconds.
However, to avoid signal fluctuations caused by air turbulence occurring between successive pulses in long range remote detection applications, it is necessary that the time interval between two successive pulses on different CO.sub.2 laser lines be less than one millisecond.
One method of meeting the time interval requirement is to use two different lasers operating at different wavelengths, triggered to produce successive pulses with nay predetermined time interval. The interval can be adjusted from microseconds to milliseconds or, if required, even longer.
However, the use of two laser systems introduces additional problems, including that of cost, that have limited its practical application In particular, if the lasers must be of relatively high intensity and operate at a pulse repetition rate of tens of hertz or higher, each laser must be provided with a gas recirculation and regeneration system, heat removal devices and other associated intricate mechanisms and components necessary to permit stable operation at such high pulse repetition rates. The necessary complexity and bulk are particularly undesirable for applications requiring field use.
A structure that combines several lasers to produce a single pulse output is described in U.S. Pat. No. 4,217,558 entitled "Pulsed Chemical Laser System". In that arrangement, a plurality of separate chemical lasers are arranged along a cylindrical path and each output pulse from the lasers is combined to form a single pulse equal to the summation of all of the individual lasers. Here, all lasers operate at the same wavelength and each laser has its own independent fuel input and output arrangement. The elements common to the laser group are a master timer for firing the lasers successively and a mirror system for combining the individual laser beams.
A laser having a physical structure somewhat similar to the one described here, but having only one optical path is described in the Review of Scientific Instruments 53(4) 1982 in an article entitled "High-repetition rate, recirculating hydrogen/deuterium fluoride laser: by R. I. Ridko, Z. Drozdowicz, and S. Linhares, and in the co-pending U.S. Patent application of Said Nazemi, Ser. No. 06/573,003 filed Jan. 23, 1984 and now U.S. Pat. No. 4,618,960 entitled "Gas Laser with Acoustic Baffle", and assigned to the same assignee as the present application.
U.S. Pat. No. 4,507,788 to James W. Barnie et al. describes a laser system capable of producing multiple pulses comprising a single optical resonator having multiple discharge regions and multiple sets of electrodes and preionizers The optical resonator is folded and the beam passes through all of the discharge regions. The system set forth in the Barnie patent, however, could not be used in he application described above and that patent suggest no modification that would enable it to meet those requirements. First, the Barnie patent only discloses a laser having a single folded resonant chamber and it is not capable of operating at two different wavelengths. Moreover, the requirement for a short interval between successive pulses cannot be met by the Barnie structure. For example, if a difference of 10 microseconds is desired between the two successive pulses, the Barnie structure cannot provide it. The first electrodes to be pulsed produce an absorbing region that persists for a significant period after after the laser pulse in that region is terminated During this period, the generation of a second laser pulse by the other pair of electrodes is prevented. The Barnie structure requires a minimum time delay of the order of 50 milliseconds to allow time for the absorbency to disappear. Even if a forced gas flow were added to the Barnie structure, which is not suggested, the minimum interval would be of the order of 0.5 milliseconds. A far shorter time interval between pulses is required for the present application.